The invention relates to a method and apparatus for the dehydration of solid porous organic material or the like, particularly for the dehydration of low rank coal, such as lignite and sub-bituminous coal, utilizing hot water drying.
In co-pending application Ser. No. 468,675 filed Feb. 22, 1983, a method and apparatus are provided for removing about 60-80 percent of the water content of the low rank coal. In the method described therein, the basic procedure is steam drying of the coal on a continuous basis.
The method and apparatus according to the present invention differ from those disclosed in said co-pending application in that instead of the utilization of direct steam heat for effecting dehydration, utilizing a single vessel, hot water is utilized to effect the dehydration, a two vessel system being provided and heating of the water being accomplished by use of a heat exchanger.
While the invention illustrated and described in said co-pending application Ser. No. 468,675 is very effective in dehydrating low rank coal, since the steam that is utilized for heat condenses on the coal, the condensate water eventually ends up going to waste when it mixes with the water of dehydration. Thus a substantial amount of fresh make-up water is necessary. Also, in the method and apparatus described in said co-pending application, the waste liquid discharged from the dehydrater, and flashed into steam, contains more heat than is desirable.
According to the present invention, a method and apparatus are disclosed which minimize consumption of fresh water relative to the method and apparatus disclosed in said co-pending application, and maximize the efficient use of the heat added to the system.
According to one aspect of the present invention, a method of dehydrating low rank coal is provided comprising the following continuous steps: Establishing a first flow stream of particulate low rank coal. Preheating the first flow stream of particles to a medium temperature, i.e. 110.degree.-120.degree. C. Immersing the first flow stream of particles in water. Transferring the particles flowing in the first stream immersed in water to a second flow stream having a pressure higher than in the first flow stream, but at a temperature less than the steam saturation temperature of the first stream. Feeding the particles in the second flow stream to the top of a heat exchange vessel, and extracting the low temperature liquid from the particles at the top of the heat exchange vessel and recirculating the extracted liquid to the second flow stream. Introducing hot wash liquid, i.e. 230.degree.-250.degree. C., adjacent but above the bottom of the heat exchange vessel to flow upwardly to further preheat coal and to accomplish high temperature energy recovery. Introducing hot liquid (e.g. 250.degree.-260.degree. C., at 700 psi) to the bottom of the heat exchange vessel, to flow downwardly therein, the water having a temperature sufficient to facilitate dehydration of the low rank coal particles. Discharging low rank coal particles from the bottom of the heat exchange vessel. Feeding the low rank coal particles discharged from the bottom of the heat exchange vessel, with hot water, to the top of a dehydration vessel, and extracting liquid from the particles at the top of the dehydration vessel and recirculating the extracted liquid to the heat exchange vessel; this liquid is heated by a heat exchanger, and provides the hot liquid introduced at the bottom of the heat exchange vessel. Effecting counter-current washing of the particles at the bottom of the dehydration vessel to cool and wash the particles at the bottom of the dehydration vessel. Withdrawing hot spent wash water, and dehydration water, from the dehydration vessel for introduction to the heat exchange vessel. Cooling the particles of low rank coal before they are discharged from the bottom of the dehydration vessel prevents break-up of the particles during depressurization, the particles being withdrawn in a third particle flow stream, by transferring the particles flowing in the third flow stream to a fourth, lower pressure flow stream. Separating the water from the particles in the fourth flow stream to flow the particles in a fifth flow stream. And, evaporatively cooling the particles flowing in the fifth flow stream to produce dehydrated low rank coal in a dry form.
According to another aspect of the present invention an apparatus is provided for dehydrating solid porous organic material or the like, such as low rank coal. The apparatus comprises: A first high pressure feeder including a low pressure first material flow loop, and a high pressure second material flow loop. A vertical heat exchange vessel having a liquid extraction means at the top thereof, a hot water liquid introduction means adjacent the bottom thereof, and a material discharge device at the bottom thereof. A conduit connecting the first high pressure feeder second loop to the extraction means at the top of the heat exchange vessel, and another conduit returning from the extraction means to the first high pressure feeder. A vertical dehydrating vessel. A conduit operatively interconnecting the material discharging device adjacent the bottom of the heat exchange vessel and a liquid extraction means at the top of the dehydration vessel. A conduit operatively interconnecting the liquid extracted from the extracting means at the top of the dehydration vessel to the hot water introduction to the bottom of the heat exchange vessel, this conduit including a heat exchanger disposed therein, and a fluid having a temperature higher than the temperature of liquid in said conduit fed to said heat exchanger. Temperature control means for sensing the temperature of liquid in the conduit including the heat exchanger, and controlling the amount of heated fluid fed to the heat exchanger in response to the temperature sensing. A means in the dehydration vessel for washing and cooling and for withdrawal of hot spent wash liquid and a conduit interconnecting said means with the heat exchange vessel to effect transfer of heat from dehydrated coal to fresh, undehydrated coal. A second high pressure feeder, including a third material flow high pressure loop, and a fourth material flow low pressure loop. A material discharge device adjacent the bottom of said dehydration vessel. A conduit interconnecting the discharge device of the dehydration vessel to the second high pressure feeder and defining a part of the third flow loop, and another conduit extending from the second high pressure feeder in the third loop and connected to a conduit for introducing water adjacent the bottom of the dehydration vessel. And, liquid-material separating means operatively connected to the second high pressure feeder by a conduit defining the fourth material low pressure loop. The liquid/material separator means operatively connected to the second high pressure feeder preferably includes a vibrating dewatering screen, a Vortex sieve, and a vibratory centrifuge.
It is the primary object of the present invention to provide an effective continuous method--and apparatus for practicing the method--for dewatering low rank coal, such as lignite and sub-bituminous coal, utilizing hot water drying, to minimize water and energy consumption. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.